1. Field of the Invention
The invention relates to radio frequency signal generation generally and more particularly to programmed sources for selecting frequencies as a function of a characteristic of an independent variable.
2. Description of the Prior Art
In the prior art, the programmable or selectable frequency source per se has a number of uses. One rather obvious application for such a device is the communications field. There a transmitter or receiver local oscillator frequency frequently must be accurately tuned through a number of discrete frequencies by an independent variable control voltage.
Other devices, such as frequency scanning radars, also require the rapid sequencing of frequency and the very accurate placement of the discrete frequencies involved.
Where a high order of accuracy and repeatability of the programmed frequencies generated by a frequency synthesizer are required, a natural choice has often been to employ banks of crystal oscillators and a switching and mixing system. The frequency stability of crystal oscillators is well known to be relatively good if temperature control and certain other precautions are employed. Because the oscillating crystal (usually quartz) utilizes the so-called piezoelectric effect, there is an actual mechanical supersonic wave or vibration involved. This makes the device subject to failure as a result of certain environmental factors and also to attritional deterioration of the quartz material of the crystal and the connections thereto. Also, the oscillating quartz crystal is subject to a relatively low upper operation frequency limitation, and accordingly where the programmable frequencies are required to be in the UHF or even microwave regions, much frequency multiplication must be employed.
The net result of such frequency synthesizer structures is therefore relatively high cost, complexity and magnification of variations.
Another known device which may be referred to as the delay-stabilized oscillator has been described in the technical literature, for example in the publication "Electronics Letters" (Mar. 9, 1972--Vol. 8, # 5). That periodical is published by the Institution of Electrical Engineers (Great Britain). A very similar device is also described in British Pat. No. 1,325,219.
Basically, this so called delay-stabilized variable oscillator operates on the principle that the delay line used provides a phase shift which is proportional to frequency. Thus a linear phase detector (which has an output linearly proportional to phase difference), when connected to measure the phase shift in the delay line will give a voltage output which is also linearly proportional to frequency. If this voltage is compared with a variable reference voltage, an error signal can be developed to change the frequency of a voltage controlled oscillator to equalize these voltages.
The output of a phase detector obviously cannot discriminate between phase shifts differing by integral multiples of 2.pi. radians, and accordingly, if the delay line used is such that many wavelengths of the input frequency are contained therein, a number of different discrete frequencies will satisfy the aforementioned criteria.
In the aforementioned prior art form of delayed-stabilized variable oscillator, a Pal (phase alternation line) is employed, this being an ultrasonic delay line, and therefore generally subject to the same type of environmental influences which influence the stability and accuracy of the ordinary crystal oscillator. It will be seen however, that the aforementioned delay-stabilized variable oscillator does however provide a significant advance in instrumentation simplicity and cost reduction.
In such systems as frequency scan radar, the instantaneous beam position is dependent on the frequency of the radar. Accordingly, a series of programmed base frequencies is required for scanning. A requirement for high angular accuracy in such a system coupled with the fact that the frequency synthesizer generating this series of frequencies is usually frequency multiplied upward to reach the microwave band employed by the radar, impose very stringent accuracy and stability requirements on the frequency synthesizer. While crystal oscillators, and Pal or SAW devices are relatively stable and reliable, they still leave much to be desired in that regard.
The manner in which the present invention develops and improves upon the prior art situation as aforesaid will be evident as this description proceeds.
In addition to the prior art aforementioned, the state of the art in respect to such light-transmissive optic fibers as are contemplated for use in connection with the present invention is summarized and explained in an article "Fiber Optic Communications: A Survey" by C. P. Sandbank, appearing in "Electrical Communication" Volume 50, # 1, 1975 a technical journal published by International Telephone and Telegraph Corporation. That article also reviews the state of the art in respect to transmittable light sources, light-beam modulation means, and appropriate demodulation devices, as will be seen to be employed in a novel combination hereinafter described. Other technical publications are also extant on these matters, the skilled practitioner being thereby able to select the appropriate detailed structure therefor.